The utility of synthetic polymers as replacement material for various types of human tissue has been substantially advanced by recent developments in improved compatibility characteristics of polymer compositions with the numerous chemical environments of the body. Although improvements in body tissue and blood compatibility (hereinafter referred to as "compatibility") have enabled more extensive use of synthetic polymers in prosthetic surgery, the continued failure of many of these implants has greatly impeded progress in treatment of persons requiring replacement of human tissue.
It has now been discovered that many of these failures were the product of mechanical mis-match, as well as compatibility rejection. Inasmuch as the effects of these two causes are quite similar--clotting of the blood and tissue rejection--the subsequent failure of the newer nonthrombogenic materials was viewed as simply a further rejection due to other chemically adverse reactions to the prosthetic implant.
Recent investigation of vascular grafts has disclosed, however, that such thrombosis and accompanying graft failure of these nonthrombogenic materials was the result of mechanical mis-match between the graft and natural tissues. Such mis-match includes a variance in elastic response and other physical properties such that the grafted material does not mechanically respond in consonance with the natural tissue. The resulting traumatization and other adverse tissue reactions cause clotting and occlusion of the vein, similar to that experienced with chemical noncompatibility.
These effects of compliance mis-match are particularly troublesome in small diameter vascular grafts. The continual variations of blood pressure cause a recurring pulsing motion resulting in constant expansion and contraction of the vascular tissues. Where the grafted material is not of an equivalent compliance with the natural vascular tissue, the inconsonant response of the grafted portion results in fluid turbulence and direct tissue damage at the sutured juncture. If the diameter of the fluid path is large or if the rate of fluid flow is high, the adverse effects of thrombosis may not be severe. This is true, for example, in the larger vessels such as the aorta which has both large diameters and substantial blood flow. If, however, these favorable conditions are not present, blood clots accumulate and frequently result in occlusion of the fluid path. For this reason, none of the previous grafts (polymer, Goretex, Dacron, or ceramic) have been effective in the venous side of the circulatory system or where the vessel diameter has been less than 6 mm on the arterial side. The combination of minimal diameter and/or reduced blood flow have precluded the effective use of synthetic material for such vascular grafts.
Because of the unavailability of suitable synthetic materials with the required compliance characteristics, vascular grafts for small diameter blood vessels and coronary bypass requirements now require the transplantation of saphenous vein from the leg of the patient or other vein material from the less critical parts of the circulatory system. The procedure is limited, however, due to potential risks of resultant circulation failure, particularly in older persons. Furthermore, it is not uncommon for an older patient to have failing saphenous veins, requiring the use of potential donors with the accompanying risks of antigenic reactions. The seriousness of these limitations is illustrated by the fact that approximately 60% of the amputations currently performed in hospitals are the result of vascular failure. A final, if perhaps sobering aspect of saphenous vein harvesting for coronary bypass is the fact that they are only available for use once.
Similarly, rejection of prosthetic materials in other body systems has been commonly experienced. Frequently, the treatment of such areas as the common bile duct, urethra, ureta and hydrocephalic tubes includes the need of tissue replacement which has previously been unsuccessful due to the concurrent needs of compatibility and mechanical compliance. Such requirements are not satisfied by synthetic materials now available in the commercial market.
Earlier polyurethane block copolymers, such as Biomer.TM. and Cardiothane.TM., are mostly in the family of thermoplastic elastomers, which are basically characterized as softening or yielding to deformation due to temperature or plastic-flow inducing stresses. Most thermoplastic polyurethane elastomers have a specific formulation or component mix ratio which produces an elastomer with essentially one set of material characteristics, i.e., no variation of characteristics compatibility. Prior vascular graft fabrication techniques (Lyman - U.S. Pat. No. 4,173,689, Kira - U.S. Pat. No. 4,725,273) sought to develop graft compliance factors required for physiologic use by means of various material elasticity producing techniques, such as controlling the dispersion of inter-wall voids or bubbles, or the formation of graft walls having both through and inter-wall pore/hole dispersion.
Clinical evidence shows that polyurethane elastomer porous structure grafts experience repeated failures due to thrombosis and inflammation (Ref. "POLYURETHANES IN MEDICINE, Cooper, CRC Press - 1986).
It should be noted that there is a substantial difference in elongation response between the compared materials. This variance becomes even more acute in the body environment where the forces to which mechanical response is required are often small. Consequently, synthetic polymers which appear to be sufficiently elastic when being manually stretched, will give little responses to very slight pressures which occur in the body. Unless the prosthesis material has mechanical properties matching such tissue to which it is to be connected, there will be an adverse reaction tending toward rejection of the material.